a(n) = 8*3^n.

Terms

a(0) =8a(1) =24a(2) =72a(3) =216a(4) =648a(5) =1944a(6) =5832a(7) =17496a(8) =52488a(9) =157464a(10) =472392a(11) =1417176a(12) =4251528a(13) =12754584a(14) =38263752a(15) =114791256a(16) =344373768a(17) =1033121304a(18) =3099363912a(19) =9298091736a(20) =27894275208a(21) =83682825624a(22) =251048476872a(23) =753145430616